This invention relates to solid oxide fuel cells (SOFCs) which are electrochemical devices that convert the chemical energy of a fuel into electricity without the use of a heat engine. In such a device operating on hydrogen, hydrocarbons, or other fuels, the change in the free energy of the cell reaction is directly available as electrical energy. The SOFCs operate with high energy conversion efficiency and very low pollutant emissions. The present-day SOFCs operate at 1000.degree. C.; research is underway to develop materials and fabrication processes that would permit operation of the SOFCs at temperatures down to 800.degree. C., or even lower.
The conventional SOFC is all ceramic in construction. It consists of a porous strontium-doped lanthanum manganite (LSM) air electrode, a dense calcia- or yttria-stabilized zirconia (YSZ) electrolyte, and a porous nickel-zirconium cermet (NZC) fuel electrode. These fuel cells are typically arranged in series in a fuel cell stack, where the individual cells are separated by a calcium or cobalt-doped lanthanum chromite (LC) bipolar separator plate (in some configurations a metallic separator plate may be used, instead). The materials provide good electrochemical activity and thermal expansion match to permit operation of the fuel cell stack at 800.degree. to 1000.degree. C. and repeated thermal cycling between room temperature and the operating temperature.
Two major types of fuel cell designs, tubular and planar, are being developed for the SOFC using the above materials. In all of these designs, the fuel (hydrogen) and oxidant (air) gases must be kept separate from each other before they react electrochemically in the fuel cell. In the tubular design being developed by Westinghouse Electric, and the planar design being developed by Technology Management, Inc., the inlet fuel and oxidant gases are kept separate from each other but the spent fuel gas is permitted to mix with and burn in the spent oxidant gas to generate the heat needed to preheat the air and maintain the high stack operating temperature. These two designs, therefore, require only minimal sealing of the gas flow passages; indeed, the Westinghouse design is commonly referred to as a seal-less design. In most other cell and stack designs, however, a gas-tight seal is required to keep the fuel and oxidant gases separate from each other, both within the stack and at the stack-to-manifold connections. Even in the Westinghouse design, the availability of a suitable sealant would permit added flexibility.
In the several different versions of the planar SOFC designs being developed, the individual cells must be arranged in a stack. To provide for the flow of the fuel and oxidant gases, while at the same time keeping the two gases separate, the edges of the cells must be sealed with a gas tight sealant. In addition, the gas supply and exit manifolds must be sealed to the stack. For instance, see the McPheeters et al. U.S. Pat. Nos. 4,761,349, the Poeppel et al. 4,476,196, the Herceg 4,476,197 and the Ackerman et al. 4,476,198. In certain designs, these gas manifolds are internal to the cell design and only supply and exhaust "tubes" need to be provided, which are made up of aligned holes in the cell structure. Even for the SOFCs with internal manifolds, effective sealants are necessary to form the appropriate gas flow tubes and channels.
Compliant sealants based on silica-based glasses have been used by some fuel cell developers. For example, Fuji Denki K.K. [Y. Harufuji, Jpn Kokai Tokyo JP 04-47, 672 (Feb. 17, 1992)]has described two types of soda-lime glasses, one solid and one liquid at 1000.degree. C. These glasses were used, with limited success in their internally-manifolded, circular planar SOFC design. Because it is a solid at 1000.degree. C. , the former glass would not be expected to tolerate thermal expansion mismatch at temperatures below 1000.degree. C. The latter glass may wick into the pores of the electrodes with time, decreasing cell performance. Both glass compositions contain highly-mobile sodium ions which can migrate into the SOFC component ceramics and degrade performance.
Dornier (see D. Stolten et al., and Spah et al., Fuel Cell Seminar Abstracts, Nov. 29-Dec. 2, 1992, pgs 253, 257) uses 2 silica-based glasses (referred to as hard and soft, respectively) in their externally manifolded, planar SOFC design. The hard glass is used to bond the manifold structure together. The soft glass is used at cell edges and at the stack-to-manifold junction. The ability to thermally cycle these structures has not yet been demonstrated.
We have also evaluated a commercial glass, Corning 0080, for possible use as sealants for the SOFC. This is a soda-lime glass, with CTE of 9.35.times.10.sup.-6 /.degree.C. Corning 0080 produced good bonds with YSZ but had unacceptable chemical and/or physical interactions with the NZC and LSM.